Process for detecting underground water



Xa; .@Wsall HS l W DERHY!! www March 7, 1961 A. s. ROGERS v2,973,811

PROCESS FOR DETECTING UNDERGROUND WTER med Nov. 2s. 1951 f Io ,f Ia "Z I6N1 I 2 HOLES FOR 1 IN SITU n o cOMBuSTION" mdx AIR INJECTION PROCESSI o l o I4 o o a o o o o o o o o v c, DE-WATEREO f ZONE.

\\ o o o I EXPLORATORY .f Of HOLES Nox -I I2 EMIL \SURFA :E RESISTIVITY TRAVERSES INV OR A. s. ROGERS I* BY Z l I v ATTORNEYS.

2,973,811 -rRocEss Fon DETECTIN'G WATER Allen S. Rogers, Bartlesville, Okla., assiguor to Phillips Petroleum Company, a corporation of Delaware Filed Nov. zs, 1957, ser. No. 698,606

9 claims. (c1.'166.4)

This invention relates to a process for detecting the location and extent of water in a permeable underground stratum containing water. A specific aspect of the invention is concerned with a method of detecting and locating the boundaries of a dewatered section of stratum containing carbonaceous Amaterial to facilitate in situ combustionl of the carbonaceous material. v

In the recovery of hydrocarbons from tar sands and other permeable strata containing combustible carbonaceous deposits by in situ v combustion, it is essential to drive the water out of the strata before effecting in situ combustion of the carbonaceous material. This is usually done by forcing air or other suitable gas into the formaf tion to drive the water back through the formation away from the injection point, or points, thereby producing an aerated zone in the rock comprising the interconnected pores therein which determine the permeability. A knowledge of the location of the boundaries of the de- 'watered zone is a valuable aid in producing the hydro! carbons from the stratum eiciently by in situ combustion.

Accordingly, it is an object of the invention to provide a process for dewatering an underground stratum and locating the boundaries of the dewatered section of the stratum. A further object is to provide a process for detecting and locating the dewatered area of a stratum containing hydrocarbon deposit and thereafter effecting in situ` combustion and recovery of hydrocarbon from the dewatered area. It is also an object of the invention to provide a process for locating a dewatered'area in a carbonaceous stratum by surface methods. Other objects Vwill become apparent from a consideration of the accompanying disclosure.

.A broad aspect yof the invention comprises making a first surface electrical resistivity survey of a selected large area overlying a, water-containing underground stratum, injecting a gas such as air into the stratum in the surveyed area through at least one borehole therein so as to force water out of a substantial section of the stratum, and thereafter making a second surface electrical resistivity survey of the selected area so as to provide data for locating the dewatered section of the stratum. The inveng tion is based upon the fact that the resistivity of porous rocks, such as are found in underground strata, depends primarily upon the conductivity or resistivity of the medium filling the pores. In the case of tar sand and other permeable strata containing carbonaceous deposits, the medium com-prises tar and, in most cases, water of variable resistivity. The resistivity of the ground water in sedimentary rocks varies from about 100 ohm-om. to 10,- 000 ohm-cm. A Venezuelan sand containing oil and water was found to have a resistivity of v7600 ohms-cm. The same sand with oil removed but containing water had a resistivity of 500 ohm-cm. This sand without either oil or water had a resistivity in the range of about 1,000 to 5,000 ohms-cm.

The procedure to be followed comprises exploring the field in which the stratum to be produced is located, by means of widely spaced drill holes extending through the UNDERGROUND,

A2 j stratum in which the surveys are to beinade` and making a surfaceresistivity survey of the area'. .The methods of making a survey are well known in the art as illustrated by the Evjen patents, U.S. 2,169,685 and U.S.

2,172,557. in making the first resistivity survey'the spacing of the electrodes is fixed to give `the approximate i depth penetration or focusing. of the current to the depth level of the carbonaceousV stratum `ras determined from analysis of cores taken from the exploration holes.

`The electrodes are spaced along parallel `traverses in the field `"area a distance approximately equal tothe depth of the stratum, and a map showing contours of iso-resistivity lines is then prepared. JThis shows the apparent resistivity` of the area to an approximate pre-determined depth.

The next step in the procedure comprises injecting air or other suitable gas into the formation through one or more boreholes so as to drive water back through the stratum away from the injection point or points. This step may take several days or several weeks depending upon the extent of the formation to be produced. vAfter the air injection step, a second surface resistivity survey 1s made along the same traverses and u-tilizing substantially the same electrode positioning as in the rst survey and a map showing lines of iso-resistivity is prepared in ac' 4cordance with the measurements obtained. Comparison watered zone and then make a third surface resistivity survey to locate the expanded boundaries of the dewatered zone. After combustion has been initiated and the combustio-n zone has been driven into the formation by either direct or inverse air injection for a substantial period, such as several days, weeks, or months, an additional surface resistivity survey can be made to determine the progress and extent of combustion. Because of the resistivity contrast between sandstone containing tar, and sandstone y conaining coke, or clean sandstone, it is feasible to locate the burned out area and the position of the combustion front as it is advancing through the stratum. Such a procedure facilitates the placement of air injection holes ahead of the fire front, in the case of inverse air injection, and provides better control of the progress of the in situ combustion process. l

In many instances, it is desirable to locate the exact level of the stratum which takes air during the air injection phase of the process. This may be done from the surfaceby means of a resistivity depth profile made at an exploratory drill hole. Such a profile is made by spacing the electrodes on a straight line on opposite sides of a drill hole, the first spacing between electrodes being less than the depth of the stratum to-be located. The electrodes are then moved a greater distance apart, each one farther from the borehole, so as to focus the current at a different depth at the drill hole. This procedure is continued until the current is successively focused at progressively greater depths until the stratum is passed. The readings of r'esistivity in each instance provide data for a rst depth profile. The next step comprises injecting or forcing air into the borehole and into the permeable stratum so as to drive water back into the stratum away from the borehole, and, after this has been accomplished, a second resistivity depth profile is made at the drill hole on the same pattern as the first depth profile and a comparison of the two resistivity profiles indicates the level at which the air "Patented Mar. 7, 19161 entered the stratum and displaced water therein. This the current is to be focused in the succeeding surface resistivity surveys made over the area of the field in the manner heretofore described.

In order to provide a more complete understanding of the invention, reference is made to the accompanying drawing which is a plan view illustrating the layout of a field in which dewatering and in situ combustion are being effected. Parallel traverse lines represent lines at the ground surface along which resistivity readings are taken at regular intervals determined by the depth of the formation. Exploratory holes 12 are made to determine the depth and extent of the formation or stratum containing the carbonaceous material and/or water to be produced.

Boreholes 14 represent air injection holes for dewatering and numeral 16 represents air injection and production holes for effecting in situ combustion in the dewatered area. 'Ihe boundary of the dewatered area is represented by numeral 18.

To'illustrate the invention, after the preliminary exploration of the field, utilizing a series of exploratory drill holes such as those shown in 12 in the drawing, a resistivity survey is made by conventional means by moving the generating and pickup electrodes (sometimes referred to as current electrodes and potential electrodes) along a traverse line 10, focusing the current to the depth of the stratum as determined in boreholes 12, or bv the denth profile technique described heretofore. and taking resistivity readings at fixed points along said line. This is repeated along the other lines in the field. In a field in which the stratum is 200 feet deep, the generating and backup electrodes are spaced substantially 200 feet apart so as to focus the current at the desired level. When the field has been covered in this manner and the resistivity readings plotted, the dewatering is commenced by injecting air or other suitable gas through boreholes 14 until the amount of injected air corresponds to the calculated area or extent of the formation which it is desired to produce for the recovery of hydrocarbons. Thereafter a second resistivity surface survey is made in the same manner as the first resistivity survey and the resistivity readings-at the various points on the chart are recorded. The differences in resistivity at certain points and in certain areas between the readings of the first survey and the second survey indicate the position of the dewatered area and the boundaries thereof. At this point the field is ready for in situ combustion and holes 16 may be drilled or utilized in any suitable pattern for either inverse or direct air injection in the in situ combustion process. The methods of recovery or combustion by in situ combustion are well known in the art and need not be disclosed in detail herein. i

The pattern of wells 16 drilled in the dewatered zone after locating the same illustrate a line drive pattern in which any row of wells in either direction may be utilized as ignition wells with production being effected through the the ignition wells by injecting air through adjacent rows of wells on either side of the row of ignition (production) wells by inverse air injection whereby the combustion fronts move toward'the injection wells countercurrently to the flow of air. The patent to Morse 2,793,- 696 illustrates inverse injection of air to move a combustion front through a carbonaceous stratum similar to that just described. It is also feasible to produce the dewatered stratum by direct injection of air through the ignition boreholes, providing the stratum is amenable to production by this method without plugging the stratum with heavy viscous hydrocarbon uidized around the combustion zone and driven into the cool stratum by the direct drive where the cooling of the fiuidized hydrocarbons causes congealing and plugging.

Another method of producing the stratum conventional in the art comprises utilizing any selected well in the d ewatered zone as an ignition well in a 5, 7, or 9-spot well pattern, driving the combustion front from the central well by direct drive, utilizing surrounding wells as production wells, or by inverse drive, utilizing surrounding wells as injection wells and the ignition well as a production well.

Traverse lines 10 may be positioned or spaced apart at any suitable distance such as 5 to 400 or 500 feet. While the spacing between the generating and pickup electrodes is determined by the approximate depth of the formation to be produced or studied, the movement of the electrodes along the traverse lines in such a spacing may be made at any selected interval such as 10 feet to 500 feet. 0f course, the closer the spacing of the traverse lines and the closer the invervals of resistivity readings along the trarverse lines, the more detailed the data for plotting the boundaries and location of the dewatered section.

In addition to determining the dewatered area for in situ combustion purposes, the method or process described herein is also applicable to ground water studies to determine zones of high connected permeability, detailed pattems of groundwater movement, land to locate areas most likely to produce water for well locations.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

l. A process for detecting dewatered areas in a permeable stratum containing carbonaceous deposit prior to producing same by in situ combustion comprising making a first surface electrical resistivity survey of a selected large area of said stratum; injecting gas into said selected area of said stratum through at least one borehole therein under non-combustion conditions so as to force water out of a substantial section thereof; thereafter makinga second surface electrical resistivity survey of said selected area so as to provide data for locating the dewatered section of stratum.

2. The process of claim l including lthe steps of measuring the dewatered section, using the data provided by said first and second electrical resistivity surveys; thereafter subjecting said section to in situ combustion so as to drive hydrocarbons therefrom; and recovering the produced hydrocarbons.

3. The process of claim l including the steps of subjecting the dewatered section, indicated by said surveys, to in situ combustion so as to drive hydrocarbons therefrom; and recovering the produced hydrocarbons.

4. The process of claim 3 wherein a third surface electrical resistivity survey of said selected area is made after a period of in situ combustion to determine the burned out area of said stratum.

5. The process of claim 1 including the steps of injecting displaceing gas into said dewatered section to displace water from the adjacent stratum, thereby substantially expanding the dewatered section; thereafter, making a third surface electrical resistivity survey of the changed selected area including the dewatered section and adjacent stratum to provide new data for locating the expanded dewatered section.

6. A process for detecting dewatered areas in a permeable stratum containing water comprising making a first surface electrical resistivity survey of a selected large area of said stratum by passing electric current through the ground between pairs of points successively located at regular intervals along parallel traverses in said area while focusing said electric current to the depth of said stra- 4turn and measuring and recording the resistivity in each instance; thereafter injecting a gas into said area through a borehole therein under non-combustion conditions so as to drive water therefrom and produce a dewatered zone in said area; thereafter repeating said resistivity survey of said area using substantially the same position and spacing of said points; and comparing the resistivity recording of the two surveys to locate the boundaries of the dewatered zone.

7. The process of claim 6 including the steps of repeating gas injection into said area after the second resistivity survey so as to expand the dewatered zone and thereafter repeating the resistivity survey to locate the expanded boundaries of said zone.

8. The process of claim 6 including the preliminary step of making a resistivity depth survey in said area by passing current through lthe ground between points spaced on opposite sides of a borehole through said stratum to obtain a resistivity measurement; thereafter successively moving the points farther from said borehole `and taking successive measurements of resistivity to focus the current to successive depths; thereafter injecting gas into the bore: hole under non-combustion conditions to force same into said stratum; and thereafter repeating said measurements from substantially the same points to locate the depth at -which water was displacel by gas in said stratum.

9. The process of claim 6, wherein said stratum contains combustible carbonaceous material, followed by the steps of initiating in situ combustion of said material in the dewatered area; driving the resulting combustion zone through said stratum; and recovering hydrocarbons driven from said stratum by the combustion and heating.

References Cited in the tile of this patent UNITED STATES PATENTS 2,241,254 Garrison May 6, 1941 2,258,614 Kendrick Oct. 14, 1941 2,770,305 Pirson Nov. 13, 1956 2,803,305 Behning et a1 Aug. 20, 1957 

